The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads and its bias control layer.
An example of a read-write head for a magnetic disk system is schematically illustrated in FIG. 1. The magnetic field that xe2x80x98writesxe2x80x99 a bit at the surface of recording medium 15 is generated by a flat coil, two of whose windings 14 can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces 12 and 13 which, while being connected at a point beyond the top edge of the figure, are separated by small gap 16. Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium 15.
The present invention is directed towards the design of read element 20 which can be seen to be a thin slice of material located between magnetic shields 11 and 12 (12 doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor 20 is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
The magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 2. In addition to a seed layer 22 on a substrate 21 and a topmost cap layer 27, these key elements are two magnetic layers 23 and 25, separated by a non-magnetic layer 24. The thickness of layer 24 is chosen so that layers 23 and 25 are sufficiently far apart for exchange effects to be negligible (the layers do not influence each others magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers 23 and 25 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 28 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 23 to 25 (or vice versa). However, once these electron xe2x80x98switch sidesxe2x80x99, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers 23 and 25 is permanently fixed, or pinned. In FIG. 2 it is layer 25 that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material, or AFM, (layer 26 in the figure). Layer 23, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface 15 of a magnetic disk).
The present invention is particularly directed to minimizing certain noise effects associated with multiple sensings of a magnetized layer. As first shown by Barkhausen in 1919, magnetization in iron can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise (BN). Long MR stripes are known to reduce this significantly for wide track recording but, for narrow-track (disk) recording, the problems of electrical signal spikes and instabilities can become serious concerns. Here, instability refers to the repeatability of output waveforms during repeated write/read cycles. The solution to this problem is to provide operating conditions conducive to single-domain films for MR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps.
One of two methods is usually used to reduce or eliminate BN. The first is to increase the effective magnetic length of the MR stripe. Long stripes exhibit reduced demagnetization at their ends and hence tend to retain single magnetic domain in the stripe. In the second method the presence of a small magnetic field in the easy axis direction of the stripe maintains a single domain and thus reduces RN. This is accomplished by depositing a longitudinal exchange bias layer directly in contact with the spin valve structure. However, a field of this sort tends to reduce the sensitivity of the sensor to a transverse field, resulting in a lower signal output. One solution to this problem that is commonly used in the prior art is to apply the longitudinal bias only in the two edge regions, as shown in FIG. 3. The longitudinally biased edge regions 38 apply a small field to sensor 30, eliminating BN while at the same time retaining the signal sensitivity of the central region. To provide this selective bias at the edges, a layer of exchange material like MnFe is deposited on the top edges of the MR-stripe. The selective longitudinal field ensures a single domain in the edge region thereby maintaining a single domain in the central section Also seen in the figure are contact leads 39.
A routine search of the prior art was conducted. While several references to various laminated structures within spin valves were encountered, none of these disclosed the exact structure or process of the present invention. Several of the references found were, however, of interest. For example, U.S. Pat. No. 5,920,446(Gill) discloses a GMR structure made up of two free layers separated by copper. These free layers are themselves laminates of two ferromagnetic layers separated by a layer of ruthenium. The latter serves as an APC (anti-parallel coupling) layer. This arrangement allows the structure to function without a pinned layer, thereby reducing its thickness.
In U.S. Pat. No. 5,998,016, Sasaki et al. incorporate an anti-diffusion layer in their spin valve structure. The preferred material for this is tantalum but ruthenium is listed as one of the alternatives. Gill et al. (U.S. Pat. No. 5,701,222) and Fontana, Jr, et al. (U.S. Pat. No. 5,701,223) both describe variations on the basic spin valve structure.
It has been an object of the present invention is to provide a synthetic antiferromagnet spin valve with controllable bias point and high output performance.
Another object of the invention has been that it should be compatible with a range of free layer compositions and thicknesses.
A further object has been that the total thickness of said spin valve be less than about 400xc3x85.
These objects have been achieved by introducing a layer of about 15 Angstroms of ruthenium between the seed layer and the free layer. This acts as an effective bias control layer with the added benefit of providing interfaces (to both the seed and the free layer) that are highly favorable to specular reflection of the conduction electrons. The HCP crystal structure of this ruthenium layer also improves the crystalline quality of the free layer thereby improving its performance with respect to the GMR ratio.